A standard image, such as an image provided by a video camera having a CCD (charge coupled device) image sensor, consists of a collection of picture elements, i.e., "pixels", where each pixel represents the brightness of a point on an object as detected at the image sensor. A range image consists of an collection of range elements, each range element representing the distance of a point on an object from a range imaging sensor. Range imaging has important industrial applications. For example, a machine vision system employing a range imaging sensor can inspect lead co-planarity of the leads of semiconductor packages in an automated quality control system.
Popular techniques for range imaging include laser triangulation and light striping. Both techniques rely on the use of an illumination source and sensing means located at different angles with respect to the object being viewed. Shifts in the sensed illumination intensity are due to height (range) differences over the surface of the object. A principal difficulty encountered with these systems is that the angled configuration unavoidably creates shadow regions which result in regions of the surface of the object where the height differences cannot be measured.
A class of range sensing techniques have been developed which employ the focal characteristics of lenses to obtain height (also range or depth) measurements. Noguchi and Nayer in "Microscopic Shape from Focus Using Active Illumination", 12th Proceedings IAPR International Conference on Pattern Recognition, pp.147-152, Jerusalem, Israel, October 1994, herein incorporated by reference, describe a system which computes depth from focus using a set of many images taken in conjunction with incremental z motion of an object, where z motion is defined as motion along the optical axis of a camera of the range sensor. This system uses projected structured (active) illumination to superimpose a strong artificial texture on otherwise smooth objects, to permit accurate sensing of best focus on such objects. The active illumination is projected onto the object through a beam splitter. Light reflected off of the object is split by the beamsplitter and focused by an imaging lens onto an electronic camera. The use of the beamsplitter provides coaxial illumination and viewing, thereby solving the shadowing problem encountered with triangulation-based systems.
In S. K. Nayer, M. Watanabe, M. Noguchi, "Real-Time Focus Range Sensor", IEEE Trans. Pattern Analysis and Machine Intelligence, vol. 18, no. 12, pp. 1186-1196, December 1996, herein incorporated by reference, there is described a range imaging sensor using active illumination and two images of the object taken at two respective focal positions. In this case, range is locally determined by comparison of the degree of defocus in the two images. This comparison is made possible by ensuring that the known spatial frequency spectrum of the projected active illumination dominates the apparent texture of the object. A beamsplitter is again employed to provide a viewing path that is coaxial with respect to the illumination path. This coaxial relationship between the illumination path and the viewing path also greatly simplifies the depth computation, since any one position in the two images is known to correspond exactly to one point at the object. However, the introduction of a beamsplitter into the optical path produces some undesirable optical effects.
Plate beamsplitters are common optical elements. A plate beamsplitter typically consists of a thin plate of flat glass, the glass plate having a thin reflective coating applied to one side of the plate. The coating thickness is chosen so that nearly half of the light incident at 45 degrees to the plate is reflected, and nearly half is transmitted, thus splitting the light into two components, the directions of the components being separated by 90 degrees. The uncoated side of the plate will unavoidably also reflect some of the incident light.
Referring to FIG. 1, the incident ray 10 is split on the first surface 12 of the beamsplitter plate 14 into reflected and refracted component rays 16 and 18, respectively. The refracted ray 18 is again partially reflected at the second surface 20, as it leaves the plate 14. The ray reflected off of the second surface is herein called the ghost ray 22, which ray is separated from the desired first reflected ray 16 by Equation 1: ##EQU1## where t is the thickness of the glass plate 14 and d is the distance separating the two rays 16, 22.
The ghost ray 22 is so named because the second surface reflection effect produces a duplicate image at the camera which is displaced by distance `d` from the desired image. For example, if the plate 14 is 3 mm thick, the ghost is displaced by 4.24 mm. Since it is the relative degree of defocus, and not absolute contrast, that is used to compute depth by focus and defocus, the ghost image creates large measurement errors in a depth by focus or depth by defocus range imaging system.
Plate beamsplitters typically have an anti-reflective coating applied to the second surface 20 of the plate 14. Using good quality broadband coatings, the reflected light ghost can be reduced in this way to around 1%. Broadband anti-reflective coatings are used because broadband illumination is preferred in range sensors to eliminate color dependency. However, due to the sensitivity of the depth by focus and defocus techniques to ghosting, large measurement errors result nevertheless.
Other types of beamsplitters are typically used when ghosts must be eliminated. The cube type beamsplitter eliminates ghosting because there is only one 45 degree surface employed. However, cube beamsplitters are expensive and introduce very thick glass wedges into the optical path. The resulting spherical aberration, due to angle-dependent refractive shifting in the thick glass, severely degrades the performance of imaging lenses, requiring provision of special compensating optics, thereby further increasing cost. Moreover, the unused backside of the cube can cause light scattering, thereby decreasing image contrast, and consequently decreasing measurement accuracy.
Pellicle-type beamsplitters consist of a nitrocellulose membrane on an optically flat frame. The membrane is coated to form a beamsplitter which is similar to the plate type, except that thickness `t` is on the order of 2 .mu.m. Thus, the ghost image is practically superimposed on the desired image. However, pellicle-type beamsplitters are much more expensive than simple plate beamsplitters, and are undesireably fragile.